The use of eddy current sensors has been found to be effective for detecting various physical, structural and metallurgical conditions in a wide variety of materials. In particular, eddy current sensors have been used in diverse applications to inspect manufactured parts.
Eddy current inspection is based on principles of electromagnetic induction. Therefore, eddy current inspection techniques are non-contact techniques in that they do not require direct electrical contact with the material or part to be inspected. Moreover, eddy current sensors are non destructive in that they do not require destruction of the part to be analyzed.
Typically, an object to be inspected is placed within or adjacent to an electrical coil in which an alternating current is flowing. As a result of the alternating current known as the driving current, eddy currents are caused to flow in the object due to electromagnetic induction. Since eddy currents oppose the primary induction current, their effects can be measured. When a crack or other defect is present on or near the object's surface, the eddy current flow is affected, which in turn causes changes in the associated electromagnetic field. The effect of the electromagnetic field can then be monitored or sensed by observing the induced voltage in one or more other coils placed within the electromagnetic field near the object's surface.
Unfortunately, currently available eddy current sensors are incapable of withstanding an aggressive environment such as one within an operational combustion turbine engine. In fact, currently available eddy current sensors are only operational up to approximately 165.degree. C. (330.degree. F.). The environment inside an operating combustion turbine engine generally varies from temperatures in excess of 165.degree. C. (330.degree. F.) to temperatures of approximately 1204.degree. C. (2200.degree. F.) depending upon the location within the turbine.
Conventional eddy current sensors are commercially available for inspecting a number of conditions in various materials or metal parts. For instance, it is desirable to monitor rotating parts of a combustion turbine engine to detect various conditions such as crack formation and displacement so that the turbine may be shut down safely, thereby avoiding further damage. However, conventional eddy current sensors are limited by their design so that they are incapable of providing a scan rate in excess of approximately 127 meters/sec. (5000 inches per second). If real time monitoring is desired for parts in a machine such as an operational combustion turbine it should be appreciated that higher scan rates are required.
Therefore, there is a need for a high resolution eddy current sensor capable of withstanding an aggressive environment which can be used in an eddy current sensing system to monitor various condition using a high scan rate so that real time monitoring can be achieved.